In the human body, proteins are involved in almost every aspect of cellular function. Proteins are linear strings of amino acids that fold and twist into specific three-dimensional shapes in order to function properly. Certain human diseases result from mutations that cause changes in the amino acid sequence of a protein which reduce its stability and may prevent it from folding properly. The majority of genetic mutations that lead to the production of less stable or misfolded proteins are called missense mutations. These mutations result in the substitution of a single amino acid for another in the protein. Because of this error, missense mutations often result in proteins that have a reduced level of biological activity. In addition to missense mutations, there are also other types of mutations that can result in proteins with reduced biological activity.
Proteins generally fold in a specific region of the cell known as the endoplasmic reticulum, or ER. The cell has quality control mechanisms that ensure that proteins are folded into their correct three-dimensional shape before they can move from the ER to the appropriate destination in the cell, a process generally referred to as protein trafficking. Misfolded proteins are often eliminated by the quality control mechanisms after initially being retained in the ER. In certain instances, misfolded proteins can accumulate in the ER before being eliminated.
The retention of misfolded proteins in the ER interrupts their proper trafficking, and the resulting reduced biological activity can lead to impaired cellular function and ultimately to disease. In addition, the accumulation of misfolded proteins in the ER may lead to various types of stress on cells, which may also contribute to cellular dysfunction and disease.
Lysosomal storage diseases (LSDs) are characterized by deficiencies of lysosomal enzymes due to mutations in the genes encoding the lysosomal enzymes. This results in the pathologic accumulation of substrates of those enzymes, which include lipids, carbohydrates, and polysaccharides. There are about fifty known LSDs to date, which include Gaucher disease, Fabry disease, Pompe disease, Tay Sachs disease and the mucopolysaccharidoses (MPS). Most LSDs are inherited as an autosomal recessive trait, although males with Fabry disease and MPS II are hemizygotes because the disease genes are encoded on the X chromosome. For most LSDs, there is no available treatment beyond symptomatic management. For several LSDs, including Gaucher, Fabry, Pompe, and MPS I and VI, enzyme replacement therapy (ERT) using recombinant enzymes is available. For Gaucher disease, substrate reduction therapy (SRT) also is available in limited situations. SRT employs a small molecule inhibitor of an enzyme required for the synthesis of glucosylceramide (the GD substrate). The goal of SRT is to reduce production of the substrate and reduce pathologic accumulation.
Although there are many different mutant genotypes associated with each LSD, some of the mutations, including some of the most prevalent mutations, are missense mutations which can lead to the production of a less stable enzyme. These less stable enzymes are sometimes prematurely degraded by the ER-associated degradation pathway. This results in the enzyme deficiency in the lysosome, and the pathologic accumulation of substrate. Such mutant enzymes are sometimes referred to in the pertinent art as “folding mutants” or “conformational mutants.”
It has previously been shown that the binding of small molecule inhibitors of enzymes associated with LSDs can increase the stability of both mutant enzyme and the corresponding wild-type enzyme (see U.S. Pat. Nos. 6,274,597; 6,583,158; 6,589,964; 6,599,919; 6,916,829, and 7,141,582 all incorporated herein by reference). In particular, it was discovered that administration of small molecule derivatives of glucose and galactose, which are specific, selective competitive inhibitors for several target lysosomal enzymes, effectively increased the stability of the enzymes in cells in vitro and, thus, increased trafficking of the enzymes to the lysosome. Thus, by increasing the amount of enzyme in the lysosome, hydrolysis of the enzyme substrates is expected to increase. The original theory behind this strategy was as follows: since the mutant enzyme protein is unstable in the ER (Ishii et al., Biochem. Biophys. Res. Comm. 1996; 220: 812-815), the enzyme protein is retarded in the normal transport pathway (ER→Golgi apparatus→endosomes→lysosome) and prematurely degraded. Therefore, a compound which binds to and increases the stability of a mutant enzyme, may serve as a “chaperone” for the enzyme and increase the amount that can exit the ER and move to the lysosomes. In addition, because the folding and trafficking of some wild-type proteins is incomplete, with up to 70% of some wild-type proteins being degraded in some instances prior to reaching their final cellular location, the chaperones can be used to stabilize wild-type enzymes and increase the amount of enzyme which can exit the ER and be trafficked to lysosomes.
Since some enzyme inhibitors are known to bind specifically to the catalytic center of the enzyme (the “active site”), resulting in stabilization of enzyme conformation in vitro, these inhibitors were proposed, somewhat paradoxically, to be effective chaperones that could help restore exit from the ER, trafficking to the lysosomes, hydrolytic activity. These specific pharmacological chaperones were designated “active site-specific chaperones (ASSCs)” or “specific pharmacological chaperones” since they bound in the active site of the enzyme in a specific fashion. Pharmacological chaperone therapy has potential advantages over ERT since a small molecule can be orally administered and may have superior biodistribution compared to protein-based therapies.
Currently, three pharmacological chaperones are in human clinical trials for Fabry disease, Gaucher disease, and Pompe disease. Since the chaperones are competitive inhibitors of the enzymes which are deficient in these diseases, appropriate dosing regimens must be designed which will result in a net increase of cellular enzyme activity and not sustained inhibition of the already-deficient enzyme.